Rn. Halthore et al., SUN PHOTOMETRIC MEASUREMENTS OF ATMOSPHERIC WATER-VAPOR COLUMN ABUNDANCE IN THE 940-NM BAND, JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 102(D4), 1997, pp. 4343-4352
Sun photometers operating in the strong water vapor absorption 940-nm
combination-vibrational band have been used to determine water column
abundance in the atmosphere when the path to the Sun is clear of cloud
s. We,describe a procedure to perform a rapid determination of the wat
er column abundance, using sun photometers with an accuracy that is ea
sily comparable to that of the radiosondes. The effect of parameters,
such as filter bandwidth, atmospheric lapse rate, and the water vapor
amount, on the accuracy of the retrieved abundance is examined. It is
seen that a narrow filter band of approximately 10-nm bandwidth, posit
ioned at the peak of absorption, is quite insensitive to the type of a
tmosphere present during calibration or measurement with less than 1%
variability under extreme atmospheric conditions. A comparison of the
retrieved water column abundance using sun photometers with contempora
neously measured values using radiosondes and microwave radiometers sh
ows that the latest version of the radiative transfer algorithm used i
n this procedure, MODTRAN-3, gives far superior results in comparison
with earlier versions because of the use of improved absorption coeffi
cients in the 940-nm bands. Results from a network of sun photometers
spread throughout the globe will be discussed.